Miyagi learned karate from his father, a fisherman in Okinawa, Japan. He worked for the richest man in the village, whose son Sato was Miyagi's best friend. In a departure from the local tradition of fathers only teaching karate to their own sons, and at Miyagi's request, Miyagi's father also taught Sato. Miyagi fell in love with Yukie, who was arranged to marry Sato. Dishonoured by their love, Sato challenged Miyagi to a fight to the death. Out of principle, Miyagi chose to emigrate to the United States without Yuki to avoid fighting Sato.

After first arriving in Los Angeles, he attended the University of California, Santa Barbara and was later interned in the Manzanar Japanese internment camp at the onset of World War II. During this time, Miyagi joined the U.S. Army and received the Medal of Honor (he was a member of the 442nd Regimental Combat Team, historically one of the most highly decorated regiments in the history of the United States Armed Forces, including 21 Medal of Honor recipients). While in the Army, he taught his Army commanding officer, Lt. Pierce, the art of karate. During his service, Mrs. Miyagi and their newborn son died in the Manzanar camp due to complications during childbirth, a loss that haunted him for decades.[9] What Miyagi did during the time between the war and the first Karate Kid film is not fully known.

In the original 1984 film, Miyagi works as a maintenance man at the South Seas apartment complex, where Daniel LaRusso and his mother have recently moved into. Later, Miyagi salvages a damaged bicycle that Daniel threw in a dumpster earlier, repairs it, and takes it back to him. Daniel is chased by bullies during a Halloween party, where Miyagi intervenes to fend off the attackers. Daniel asks Miyagi to teach him karate after Miyagi saves him. At first, Miyagi refuses, but, after meeting the rambunctious and merciless John Kreese, a former special forces veteran running the Cobra Kai dojo, Miyagi agrees to teach Daniel karate. The two soon learn that there will be a tournament, where the Cobra Kai students, in which some bully Daniel at school, will be competing. At the start of Daniel's karate training, Miyagi assigns him to do several non-karate related house chores. On Daniel's first day of training, he waxes a number of cars. On the second day of training, Daniel sands the wooden floors of Miyagi's house. On the third day, Daniel paints the fences. On the fourth day, Daniel paints Miyagi's house. At this point, Daniel gets upset and threatens to leave. Miyagi then shows him how the chores were in fact training Daniel to do karate blocks. After the chores, Miyagi continues to train Daniel, and their bond develops. Miyagi soon confesses to Daniel about the dual loss of his wife and newborn son due to complications arising from childbirth at Manzanar internment camp while he was serving with the 442nd Infantry Regiment during World War II in Europe, where he received the Medal of Honor. Miyagi teaches Daniel important life lessons such as the importance of personal balance, reflected in the principle that martial arts training is as much about training the spirit as the body. Miyagi accompanies Daniel at the tournament, where Daniel wins the finals.

In Part II, Daniel has recently won the tournament. Miyagi humiliates John Kreese in the parking lots, causing him to suffer bleeds on both hands after punching the glass windows on the cars. Six months later, Miyagi receives a letter, explaining that his father is dying due to his very old age. Miyagi explains that he once fell in love with a woman named Yukie, who was arranged to marry his best friend, Sato, son of the richest man in the village and fellow karate student of his father. Upon announcing his intentions to marry Yukie, Sato challenged him to a fight to the death. Rather than fight, however, Miyagi left Japan as a show of self-imposed exile. Miyagi and Daniel travel to Okinawa, where they are greeted by Chozen Toguchi, who drives Miyagi and Daniel to one of Sato's warehouses, and reveals himself to be Sato's nephew. Sato tries to goad Miyagi into a fight to restore his honor, but Miyagi turns it down. Once at the village, Miyagi and Daniel are welcomed by Yukie and her niece, Kumiko. The two reveal that Soto owns the village's land title, and are forced to rent property from him. Yukie reveals that she never married Sato, because of her love for Miyagi. After Miyagi's father dies, Sato gives him three days to mourn out of respect before their fight. Miyagi then shows Daniel the secret to his family's karate – a handheld drum that twists back and forth illustrating the "drum technique", a block-and-defense karate move that Daniel begins to practice. Miyagi's family property is vandalized by Chozen and his crew, which are fended off when Miyagi intervenes. Miyagi and Daniel intend to return to California before the situation worsens. However, Sato shows up with bulldozers, threatening to destroy the village if Miyagi refuses to fight. Forced to comply, Miyagi gives in on the condition that Sato signs the village's land title over to the villagers, regardless of the fight's outcome. Sato initially balks, but agrees after Miyagi describes the condition as a "small price" to pay for honor. On the day of their fight, a typhoon strikes the village, where Sato is trapped under the ruins of a dojo that was destroyed by the storm. Miyagi and Daniel rescue him, and take him to a nearby shelter, where other innocent bystanders witness a child stuck on top of a bell tower, soon rescued by Daniel. The next morning, the bulldozers help rebuild the village, while Sato hands over the village's land title and ask for Miyagi's forgiveness, who accepts. Miyagi and others attend an O-bon festival at the castle of the village, where a vengeful Chozen takes Kumiko hostage and challenges Daniel to a fight to the death. Miyagi uses a handheld drum, and so do all the others in the festival, to distract Chozen, giving Daniel a chance to subdue him. Kumiko is freed afterward, while Miyagi looks on.

In Part III, Miyagi and Daniel return to California, but witness that the South Seas apartment complex is being demolished, leaving Daniel homeless and Miyagi unemployed. Miyagi offers Daniel to stay at his house. Daniel uses his college funding to help Miyagi open up a nursery shop for bonsai trees. Miyagi thanks Daniel and makes him a partner for the new business. While their newfound business is in progress, they are unaware that John Kreese is attempting to resurrect, and his longtime comrade, Terry Silver, has been gathering information out of the two who are running the business, and plot to goad Daniel into participating in the next upcoming tournament, but Daniel announces that we will not be attending to defend his title. Miyagi continues to train Daniel with new techniques, until Silver approaches the two with a fake story, requesting for forgiveness for Kreese's previous behavior. Mike Barnes, a top prospect for the tournament, enters the frey, causing a ruckus with Daniel, until Miyagi intervenes, fending him and his neighbors off. The two then witness that the bonsai trees have disappeared from the shop, with an application for the tournament replacing, causing Daniel's concern to grow even more uneasy. It is revealed that Miyagi had a valuable bonsai tree he brought from Okinawa, where Daniel and his newly befriended neighbor, Jessica Andrews, decide to plant the tree half way down a tree in hopes of selling it. However, the bullies return while Daniel and Jessica ascend uphill, leading to Barnes breaking the tree, and forcing Daniel to compete in the tournament. Daniel takes the broken bonsai tree to Miyagi, who then confesses that he sold his truck in order to obtain a new stock of trees, and explains that he now cannot train Daniel. Daniel goes through a series of unfortunate events, training under Silver's wing, and attacking a man at a nightclub, but apologizes to Miyagi and Jessica soon afterward. Still refusing to compete in the tournament, Daniel is tricked by Silver's true agenda, with Barnes and Kreese coming in, while Miyagi intervenes, and agrees to train Daniel once again. The two replant the now-healed bonsai tree. At the tournament, Daniel defeats Barnes as Miyagi looks on, while Silver and Kreese are not pleased, speculating that Cobra Kai is finished forever.

In The Next Karate Kid, Miyagi travels to Boston, Massachusetts to attend a commendation for Japanese-American soldiers, who fought in the 442nd Regimental Combat Team during World War II. He meets Louisa Pierce, the widow of his commanding officer, Lieutenant Jack Pierce, and they explain about their past war stories. Miyagi meets Julie, a teenage girl plagued with anger issues, who also reveals that her parents died in an automobile accident. One day, Miyagi invites Louisa to his house in Los Angeles to have a moment of silence, while Miyagi stays behind at Boston to act as Julie's caretaker. Julie has a series of friction during her school days, and is nearly struck by a car when jumping into a tiger position. Julie reveals to Miyagi that she learned karate from her father, who was also taught the same thing by her grandfather, and the grandfather was once Miyagi's student. After Julie getting arrested as well as a two-week suspension from the school, Miyagi uses this time to take Julie to a Buddhist monastery to teach her the true ways of karate and how to handle her anger issues. Miyagi teaches Julie lessons about balance, co-ordination, awareness and respect for all life. As Julie is preparing for her high school prom, Miyagi teaches her how to dance and purchases her a dress. Julie attends the prom, while Miyagi goes bowling with the Buddhist monks. Still, things goes haywire as Julie, along with her friend, Eric McGowen, are under siege by Colonel Dugan and the Alpha Elite. After Eric's car is set on fire, Eric is saved by Julie and Miyagi. The fiasco continues as Miyagi challenges Colonel Dugan to a fight, and easily defeats him.

Mr. Miyagi has a deep philosophical knowledge of life and has extraordinary martial arts skill.[10] In the second film, Mr. Miyagi explains that he is descended from Shimpo Miyagi, who was very fond of both fishing and sake. One day in 1625 while fishing and very drunk, he passed out on his fishing boat off the coast of Okinawa and ended up on the coast of China. Ten years later, Shimpo returned to Okinawa with his Chinese wife, his two kids, and the secret of Miyagi family karate. This implies that Shimpo Miyagi, like many Okinawan karate masters, was trained in Chinese martial arts during his stay in China. The secret of the Miyagi family karate appears to be a Den-den daiko, on which the drum technique is based. Other aspects of the style involve the crane technique, breathing technique, techniques from the kata Tensho (wax on wax off) and the kata Seiunchin.

Mr. Miyagi trains the player and is a hidden unlockable fighter in the game Reality Fighters for PS Vita. He wears his trademark outfit, uses a fishing pole as a weapon, repeatedly mentions his bonsai-growing, and makes subtle references to Karate Kid, including the fly-and-chopsticks exercise.